1. Field
The following description generally relates to encoders and decoders and, in particular, to a computationally-efficient filterbank structure for MPEG family of audio codecs, such as High-Efficiency Advance Audio Coding (HE-AAC), Enhanced Low-Delay (ELD), Spatial Audio (SAOC) and Unified Speech and Audio (USAC) codecs.
2. Background
One goal of audio coding is to compress an audio signal into a desired limited information quantity while keeping as much as the original sound quality as possible. In an encoding process, an input audio signal in a time domain is transformed into a frequency domain audio signal, and a corresponding decoding process reverses such operation by transforming the frequency domain audio signal to an output audio signal in the time domain.
Audio codecs may be based on modeling the psychoacoustic characteristics of the human auditory system. For instance, the audio signal may be split into several frequency bands and the masking properties of the human ear may be used to remove psychoacoustical redundancies. Hence, audio codecs generally rely on transform coding techniques for compression. Audio codecs typically suitable for coding any generic audio material at low bit rates. Since audio codecs operate on longer frame lengths for good frequency selectivity and also since they generally use orthogonal filterbanks, their round-trip algorithmic delay is high, making them unsuitable for full-duplex communications. However, the need for high quality, low bit rate, full-duplex audio communication applications (such as audio and video conferencing) is growing.
MPEG standardized a low delay audio codec called MPEG-4 Advance Audio Coding (AAC)-Enhanced Low Delay (ELD) that attempts to improve coding efficiency while keeping the codec delay low enough for full-duplex communications. AAC is a wideband audio coding algorithm that exploits two primary coding strategies to reduce dramatically the amount of data needed to represent high-quality digital audio. First, signal components that are perceptually irrelevant are discarded and, second, redundancies in the coded audio signal are eliminated.
Coding efficiency is increased by using Spectral Band Replication (SBR). To minimize the delay introduced, a low-delay version of the analysis and synthesis SBR filterbanks is used. Often, these audio codecs may operate on mobile platforms where processing power and battery life are limited. Hence, there is a need for fast algorithms for the computationally intensive operations of an audio codec. Typically, filterbanks and transforms contribute a significant part of the computational complexity. For the low-delay SBR (LD-SBR) filterbanks used in AAC-ELD, the corresponding matrix multiplication operation may be one of the most computationally intensive parts.
Thus, a computationally efficient filterbank is needed to reduce computational complexity and/or delay in audio codecs.